(1) Field of the Invention
This invention relates to an electroconductive polymer composition. More particularly, it relates to an electroconductive polymer composition for preventing electromagnetic interference (hereinafter referred to as "EMI") and an accumulation of static electricity.
(2) Description of the Related Art
With the development of electronic appliances such as a computer, the prevention of EMI and an accumulation of static electricity have become important, and various processes therefor have been proposed. Typical examples of known EMI prevention processes are a surface-treating process in which only the surface of a non-electroconductive material is treated, for example, coated with an electroconductive substance; an internal treating process in which an electroconductive filler is dispersed in or laminated on a non-electroconductive material; and a metal process in which a metal having a high electroconductivity is used.
Note, an electroconductive polymer composition comprising a thermoplastic resin and an electroconductive filler dispersed in the resin is known.
Of these conventional techniques, many reports have been made on an electroconductive polymer composition comprising an electroconductive filler dispersed in a polymer.
In the usual process for the preparation of an electroconductive polymer article, a filler is first mixed and kneaded with a matrix resin and the mixture is shaped by injection molding or the like. A banbury mixer, a mixing roll, a twin-screw extruder or the like is used for the kneading.
If the amount of the filler incorporated is increased, to improve the electroconductivity, the melt fluidity is reduced and the molding and processing become difficult. Furthermore, even if molding is possible, because of a short shot or the like, a satisfactory shaped article cannot be obtained, and often the resulting shaped article has poor mechanical properties, such as a poor impact strength.
The incorporation of excessive amounts of some fillers is not advantageous from the economical viewpoint.
The scatter of the electroconductivity of products is large, due to the processing conditions, and in practical operation, it is often found that the electroconductivity of the products differs. Moreover, in some cases, even if a good electroconductivity is obtained just after the preparation, the electroconductivity is gradually lost with the lapse of time.
The present inventors collectively reported on the series of investigations they made, in the "Journal of the Adhesion Society of Japan, Vol. 23, No. 3, pages 103-111 (1987)", and in this report showed that the interfacial affinity between the polymer and the filler is an important factor having an influence on the manifestation of the electroconductivity, the behavior of the electroconductivity in various resin matrices can be elucidated based on the difference in the dispersion state of the filler, and the volume fraction (hereinafter referred to as "Vf") of the filler producing the electroconductivity by a formation of conducting paths differs greatly according to the kind of resin matrix. Furthermore, by changing the dispersion state of fine particles of carbon black by utilizing a thermal relaxation of polymers, it was found that the electroconductivity was improved by an aggregation of the filler.
When preparing an electroconductive polymer composition by dispersing an electroconductive filler into a polymer, if an attempt is made to improve the electroconductivity by increasing the amount of the electroconductive filler, a deterioration of properties other than the electroconductivity, such as the processability and mechanical properties, and an increase of the costs, cannot be avoided.